Method and devices for producing air sensitive electrode materials for lithium ion battery applications

ABSTRACT

A unit for use within a furnace which is absent a controlled atmosphere, for carrying out a synthesizing process for synthesizing precursors to form a synthesized product at elevated temperatures. The unit consists of a vessel, having at least one opening, for containing materials of the synthesizing process, and a solid reductive material. The materials of the synthesizing process are separated from the atmosphere of the furnace by either the vessel or the reductive material. The unit is especially suited for synthesizing LiFePO 4  from Fe 2 O 3 , Li 2 CO 3 , carbon black, and phosphoric acid precursors.

FIELD OF THE INVENTION

The present invention is concerned with reaction chambers to be utilizedfor the mass production of air sensitive materials, especially for thesynthesis of electrode materials for lithium batteries.

BACKGROUND OF THE INVENTION

Oxidation and reduction reactions are commonly utilized for thesynthesis of inorganic crystalline materials. This is especially truefor the synthesis of electrode materials for Li-ion batteries includingcathode and anode materials. Conventionally, cathode materials such aslithium cobalt oxide, lithium nickel oxide, lithium manganese oxide andthe mixed oxides are synthesized under oxidative environments. Thesematerials are more readily obtainable since control of an oxidative heattreatment environment (e.g. heat treatment in open air environment) isnot difficult. In contrast, a reductive environment is less feasiblesince control of a reductive heat treatment atmosphere is difficult. Thedifficulty stems from the fact that during the heat treatment steps ofthe synthesis, especially at elevated temperatures (e.g. >500° C.), aslight leakage of air during the heat treatment would be detrimental forthe reaction and therefore degrade the quality of the synthesizedmaterials. The difficulties in controlling a reductive atmosphere makemass production unlikely or very expensive. One example is the synthesisof lithium iron phosphate that is conventionally synthesized in areducing or inert atmosphere. A LiFePO₄ type cathode material has beendiscussed for replacing LiCoO₂ for lithium ion battery applicationsbecause of the potentially lower cost (Fe replacing Co) and the saferoperating characteristics of the material (no decomposition of thematerial during charging). However, processing issues such as hightemperature heat treatment (>600° C.) under an inert or reducingatmosphere makes the material expensive and it is not widely accepted.Until the present, the maintenance of a reducing or an inert atmosphereat a high temperature was still a key factor limiting good control ofthe quality of the synthesized materials. To ensure a complete seal ofthe furnace, especially when heat treated at high temperatures, is verydifficult.

Prior arts such as U.S. Pat. Nos. 5,910,382, 6,723,470, 6,730,281,6,815,122, 6,884,544, and 6,913,855, in general, teach methods andprecursors utilized for the formation of stoichiometric LiFePO₄, or thesubstitution of cations for iron. The above mentioned patents only showhow the materials are synthesized. None of the prior art teaches how tocontrol the heat treatment environment efficiently and cost effectively.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide methods and devicesfor controlling a heat treatment environment that can be widelyapplicable to the synthesis of materials to form electrode materials. Itis a further object of the invention to provide methods and devices thatare cost effective and insure good quality of the synthesized material.

SUMMARY OF THE INVENTION

The present invention is a unit, for use within a furnace absent acontrolled atmosphere, in a synthesizing process for synthesizingprecursors to form a synthesized product at elevated temperatures. Theunit has a vessel, having at least one opening, for containing materialsof the synthesizing process, and a solid reductive material, wherein thematerials of the synthesizing process are separated from the atmosphereof the furnace by either the vessel or the reductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily apparent from the followingdescription of preferred embodiments thereof shown, by way of exampleonly, in the accompanying drawings, wherein:

FIGS. 1( a) and 1(b) are illustrations of a first embodiment of the unitof the invention;

FIGS. 1( c) and 1(d) are illustrations of a second embodiment of theunit of the invention;

FIG. 1( e) is an illustration of a third embodiment of the unit of theinvention.

FIG. 2( a) is an illustration of units of the first and/or secondembodiments in a furnace for carrying out a synthesizing process;

FIG. 2( b) is an illustration of units of the third embodiment in afurnace for carrying out a synthesizing process;

FIG. 3 is a graph of an x-ray diffraction pattern for a representativesample of a synthesized electrode material prepared using units of theinvention;

FIG. 4 is a graph for showing battery test data for the same material asin FIG. 3;

FIG. 5 is a graph of x-ray diffraction patterns for 5 similarsynthesized electrode materials prepared using units of the invention;and

FIG. 6 is a graph for showing battery test data for 10 similarsynthesized electrode materials prepared using units of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1( a)-1(eshow schematic diagrams of individually sealed units(ISU) containing materials that are subjected to the synthesizing heattreatments. Designs of furnaces that contain the ISUs of differentgeometries are shown in FIGS. 2( a) and 2(b).

In FIGS. 1( a) and 1(b) the ISU 1 is a vessel having one end 2completely sealed while the other end 3 is open to the atmosphere.Precursors to be synthesized to form an electrode material are containedat 4. The precursors, intermediate products, and resulting material ofthe synthesizing process are referred to as materials of thesynthesizing process throughout the description. The materials of thesynthesizing process, contained at 4, are protected from the atmosphereof the furnace, into which ISUs are placed for heating, by either thematerial of the vessel 1, or a solid reductive material layer 5 thatlimits the permeation of air from the furnace atmosphere. It should bementioned that since the reductive material (e.g. carbon black) isusually porous, the porosity of the reductive material layer would allowthe permeation of any gas by-product released from the material beingsynthesized, to the atmosphere. In general, either the gas by-product orthe oxidation of the reductive material would generate gas and keep thepressure within the ISU positive, compared to the atmosphere. However,if the material being synthesized does not generate gas as a by-product,a decrease of the porosity of the reductive material layer (by means oftapping, for example) would ensure separation from the atmosphere.

In FIGS. 1( c) and 1(d) each ISU of a second embodiment is a vessel 1having both ends 6 open to the environment. Precursors to be synthesizedto form an electrode material are contained at 4. The materials of thesynthesizing process, contained at 4, are protected from the atmosphereof the furnace, into which ISUs are placed for heating, by solidreductive material layers 5 that limit the permeation of air from thefurnace atmosphere. As mentioned above, the solid reductive material isusually porous to allow permeation of any gases resulting from thesynthesizing process.

In both of the embodiments, a divider 11 can be used to separate thereductive material 5 from the material 4 of the synthesizing process.The divider preferably is inert to the materials being separated andporous to any gases being generated. Also, as shown in FIGS. 1( a)-1(d),at 7, a high-temperature durable glass fiber packing can be used to holdall of the materials in the vessels .

Similar characteristics can be observed in a third embodiment of an ISUshown in FIG. 1(e). From FIG. 1( e), it can be seen that the materialsto be synthesized 4 are contained in a crucible 8. The path of airflowfrom any open side of a vessel 9 is controlled by the presence ofreductive material 10. A bottom of the crucible separates the reductivematerial from the materials of the synthesizing process. A tray 12facilitates handling of the unit. Vessel 9 is not sealed tightly againsttray 12 in order that gases can flow freely to or from the reductivematerial, as shown at 18.

FIGS. 2( a) and 2(b) show the various embodiments of the invention asutilized in a furnace to carry out the synthesizing process.

In FIG. 2( a) first embodiments and/or second embodiments are shown infurnace 13. Heating elements of the furnace are shown at 14.

In FIG. 2( b) four units of the third embodiment of the invention areshown at 15 in furnace 16. Heating elements of the furnace are shown at17. As mentioned above, the furnaces are not required to be sealed and acontrolled inert or reducing environment is not necessary.

The common structures of the ISUs are as follows:

-   -   a. An ISU includes a space that contains the materials being        subjected to the synthesizing heat treatment;    -   b. An ISU includes a space that contains the reductive material;    -   c. The reductive material is placed in the vessel in a manner        as:

Uncontrolled atmosphere/reductive material/synthesized material (FIGS.1( a) and 1(b)), or

uncontrolled atmosphere/reductive material/synthesized material/reductive material/uncontrolled atmosphere (FIGS. 1( c) and 1(d));

-   -   d. The reductive material can be placed on top of the        synthesized material as shown in FIGS. 1( a)-1(d) or somewhere        else in contact with the outer atmosphere as shown in FIG. 1(        e);    -   e. The ISU can dissipate gas generated by the synthesizing        reaction.

In the embodiments of FIGS. 1( b) and 1(d) the flow of gases is from thematerials of the synthesizing process, through the reductive material tothe uncontrolled atmosphere, or the reverse of same.

In the embodiments of FIGS. 1( a) and 1(c) the flow of gases is from thematerials of the synthesizing process, through the separator, throughthe reductive material to the uncontrolled atmosphere, or the reverse ofthe same.

In the embodiment of FIG. 1( e) the flow of gases is from the materialsof the synthesizing process, through the separation between the crucibleand the vessel, through the reductive material to the uncontrolledatmosphere, or the reverse of same.

Other advantages provided by the utilization of ISUs include:

-   -   A. No need for an inert atmosphere in the furnace, thus        resulting in:        -   i. Easy scale up for production;        -   ii. Much lower cost of a furnace since a gas-tight furnace            becomes unnecessary;        -   iii. The cost of inert gas can be saved;        -   iv. Overall cost of the synthesis protocol is reduced; and        -   v. Easy control of the quality of the resultant synthesized            materials.            -   Since one ISU can be considered as one furnace.    -   B. Good performance of the synthesized material as demonstrated        in the following examples.    -   C. Consistency in performance of the synthesized material, which        is extremely important for battery applications.

Owing to the advantage of the controlled heat treatment environmentprovided by the ISUs, materials that require heat treatment under aninert atmosphere can be obtained easily and cost efficiently. Followingare examples of materials synthesized in an ISU of the invention, inorder to better describe use of the invention.

EXAMPLE 1. Synthesis of LiFePO₄ using Methods and Devices of theInvention

In order to demonstrate the novelty of the ISUs disclosed in the presentpatent application, the synthesis of conventional LiFePO₄ in bulkquantity is used. 12 kg (75 moles) of Fe₂O₃ and 5.55 kg (75 moles) ofLi₂CO₃ and 1.8 kg (150 moles) of Super P (carbon black, available fromMMM Carbon, Belgium), molar ratio of (1:1:2), were mixed together withthe addition of a suitable amount of water to form a paste. After mixingthoroughly, the proper stoichiometric amount of phosphoric acid wasadded and extended mixing was utilized (6 hours). Finally, the slurrywas dried in air at 150° C. for 10 hours, followed by further heattreatment at 400° C. for 10 hours until chunks of materials wereobtained. The as-prepared material was then subjected to grinding andball milling for about 12 hours. The ground powdery materials was thenloaded into several ISUs as shown in FIG. 1( a) with the addition of acarbonaceous material placed directly on top of the ground powderymaterial for heat treatment. In practice, the carbonaceous material canbe placed directly on top of the synthesized material or separated by athin layer of porous glass fiber fabrics or other inert plate. The ISUswere then placed in a furnace as shown in FIG. 2( a).

The heat treatment was conducted at 650° C. for 24 hours resulting inthe synthesized material. After the heat treatment step, slight grindingand sieving were conducted on the synthesized material. The post-heattreated materials were then ready for further tests, as will bedescribed below.

The utilization of ISUs is not limited to the synthesis of lithium ironphosphate, or limited to the choice of starting materials and precursorprocessing steps described for the synthesis of lithium iron phosphateof the present example.

X-ray diffraction pattern data of the synthesized material is shown inFIG. 3. It is observed that phase pure material was obtained using theprocessing methods and devices presented in this example, without theuse and control of an inert gas, such as nitrogen or argon. Battery testdata (obtained using a three electrode design test battery and lithiumis utilized as the reference electrode) are shown in FIG. 4. From FIG. 4it can be seen that the capacity is high during the firstcharge-discharge cycle (˜C/5 rate, 0.23 mA/cm²). The materialsynthesized in the present case is comparable or superior to the priorart material disclosed in U.S. Pat. No. 6,723,470, which was obtainedusing an inert atmosphere as a heat treatment environment.

EXAMPLE 2 Demonstration of Consistently the Synthesized LiFePO₄ usingMethods and Devices of the Invention

In the present example, ten batches of materials synthesized using theISUs shown in FIG. 1( a) were tested for quality consistency. Theprecursor processing procedures for each batch were the same as theprocedures described in example 1. The ten different batches weresubjected to 10 identical heat treatment procedures in ISUs. From theten batches, five batches were subjected to the x-ray diffractionpattern analyses and the results are shown in FIG. 5. Also, a stack ofthe 1^(st) cycle data for each batch is shown in FIG. 6. More accuratenumerical data is provided in Table 1. From FIG. 5 it can be seen thatall of the materials are phase pure in nature. The peak intensity andpeak positions for each sample are similar, as shown and indicated inFIG. 5. In FIG. 6, the 1^(st) charge and discharge plot for each sampleis again very similar. The 1st charge capacity ranges from 132˜137 mAh/gand the 1^(st) discharge capacity ranges from 118˜124 mAh/g. All thesedata suggest that the consistency of the materials synthesized using theISUs is insured.

TABLE 1 The detailed electrochemical data of the ten batches heattreated using the ISUs. 1^(st) 1^(st) charge 1^(st) charge dischargeaverage 1^(st) discharge 1^(st) cycle Batch capacity capacity voltageaverage Coulomb Names (mAh/g) (mAh/g) (V) voltage (V) efficiency AE11021133.97 118.69 3.5083 3.3800 0.8859 AE11031 132.15 118.64 3.5070 3.38050.8978 AE11041 137.30 124.11 3.5016 3.3845 0.9039 AE11051 135.29 118.603.5088 3.3778 0.8766 AE11061 133.03 119.06 3.5066 3.3810 0.8950 AE11121132.14 118.75 3.5071 3.3608 0.8987 AE11131 133.19 120.19 3.5083 3.37910.9024 AE11141 135.69 122.59 3.5189 3.3794 0.9035 AE11151 136.43 122.553.5109 3.3776 0.8983 AE11161 134.71 120.52 3.5090 3.3778 0.8947

The devices of the present invention provide the following advantages.There is no need for the use of an inert gas in the furnace, such asnitrogen or argon, or forming gas (nitrogen plus hydrogen), thus acompletely sealed furnace is not required. The ISUs are semi-open to theatmosphere of the furnace, thus sealing of the ISUs is not difficult.There is a short thermal diffusion distances from the heat source to thematerial being synthesized. With use of the reductive material, such ascarbon black or carbonaceous materials for air permeation prevention,even if a small amount of air permeation occurs during heat treatment,oxidation of the carbonaceous material prevents further oxidation of thematerial being synthesized. The reductive material can be porous so toallow the dissipation of gas produced by the materials that aresubjected to the heat treatment. The depth of the ISUs shown in FIG. 1(a) and 1(b) are adjustable for the prevention of oxidation, for examplea longer depth would give a better-isolated environment. Also, thegeometry of the ISUs is flexible to accommodate the design of thefurnaces, such as shown in FIGS. 2( a) and 2(b).

While specific materials, dimensional data, etc. have been set forth forpurposes of describing embodiments of the invention, variousmodifications can be resorted to, in light of the above teachings,without departing from applicant's novel contributions; therefore indetermining the scope of the present invention, reference shall be madeto the appended claims.

1. A system for use in a synthesizing process for synthesizingprecursors to form a synthesized product at elevated temperatures,comprising a furnace having a furnace chamber open to the atmosphere forheating materials disposed in the furnace chamber, materials of thesynthesizing process, gases within the furnace chamber consistingessentially of gases resulting from heating the materials of thesynthesizing process and air of the atmosphere entering the furnacechamber, a solid reductive material porous to gases generated by thesynthesizing process, a vessel disposed in the furnace chamber, thevessel having at least one opening with said at least one opening beingfully open to gases within the furnace chamber so as to enableunrestricted flow of gases, for containing the materials of thesynthesizing process and the solid reductive material , said solidreductive material being disposed across any opening in the vessel, andgases within the vessel consisting essentially of gases resulting fromheating the materials of the synthesizing process and air from theatmosphere, wherein said materials of the synthesizing process arecompletely surrounded by at least one of the vessel and the solidreductive material so as to be between said materials of thesynthesizing process and the air of the atmosphere within the furnacechamber, a flow of gases is from the materials of the synthesizingprocess, through the solid reductive material to the furnace chamber, orthe reverse of same, the materials of the synthesizing process compriseFe₂O₃, Li₂CO₃, carbon black, and phosphoric acid, the materials areheated to a temperature greater than 600° C., and the synthesizedproduct is LiFePO₄.
 2. The system of claim 1, wherein said vessel andsaid reductive material are arranged such that said materials of thesynthesizing process are in contact with said solid reductive material.3. The system of claim 1, further comprising a divider for separatingthe materials of the synthesizing process from the solid reductivematerial, wherein the divider is of a material substantially inert tothe materials being separated.
 4. A system for use in a synthesizingprocess for synthesizing precursors to form a synthesized product atelevated temperatures, comprising a furnace having a furnace chamberopen to the atmosphere for heating materials disposed in the furnacechamber, materials of the synthesizing process, gases within the furnacechamber consisting essentially of gases resulting from heating thematerials of the synthesizing process and air of the atmosphere enteringthe furnace chamber, a solid reductive material porous to gasesgenerated by the synthesizing process a vessel disposed in the furnacechamber, the vessel having at least one opening which is open to gaseswithin the furnace chamber, for containing the materials of thesynthesizing process and the solid reductive material, said solidreductive material being disposed across all the openings in the vessel,gases within the vessel consisting essentially of gases resulting fromheating the materials of the synthesizing process and air from theatmosphere, and a crucible, disposed within said vessel, for holdingmaterials of the synthesizing process said crucible being between thematerials of the synthesizing process and said vessel and said solidreductive material, said crucible having at least one opening, with theat least one opening being fully open to gases within said vessel so asto enable unrestricted flow of gases, wherein said materials of thesynthesizing process are completely surrounded by at least one of thevessel and the solid reductive material so as to be between saidmaterials of the synthesizing process and the air of the atmospherewithin the furnace chamber, a flow of gases is from the materials of thesynthesizing process, through the solid reductive material to thefurnace chamber, or the reverse of same, the materials of thesynthesizing process comprise Fe₂O₃, Li₂CO₃, carbon black, andphosphoric acid, the materials are heated to a temperature greater than600° C., and the synthesized product is LiFePO₄.
 5. The system of claim1 or 4, wherein the solid reductive material is porous to gasesresulting from oxidation of the solid reductive material.
 6. The systemof claim 3, wherein the solid reductive material is porous to gasesresulting from oxidation of the solid reductive material, and thedivider is porous to gases resulting from the synthesizing process. 7.The system of claim 5, wherein a combination of the porosity andseparating thickness of the solid reductive material substantiallyprevents gases of the furnace chamber from entering the synthesizingprocess.
 8. The system of claim 1 or 4, wherein the solid reductivematerial has a separating thickness of 5-10 centimeters.
 9. The systemof claim 1 or 4, wherein the solid reductive material is carbon black,coal, coke or metal powder.
 10. The system of claim 9, wherein the solidreductive material is carbon black.
 11. The system of claim 1 or 4,wherein the vessel is of a material substantially inert to the materialsof the synthesizing process and the solid reductive material.
 12. Thesystem of claim 11, wherein the material of the vessel is stainlesssteel.